U.S. Pat. No. 4,446,941 to Laurich-Trost which issued May 8, 1984 entitled "Steering System for Utility Vehicle" discloses a utility vehicle which can be operated in a number of different steering modes depending upon the desired use of the vehicle. Four different steering modes are suggested.
The utility vehicle disclosed in the '941 patent has a fork lift at one end and a hydraulically activated boom at an opposite end for performing material handling, excavating, and other utility vehicle tasks. In addition, the vehicle includes an outrigger mechanism which can be selectively used to increase the stability of the vehicle during work operations.
Use of the utility vehicle for a number of different operations such as excavating and the like have resulted in a multiple steering mode design that provides the vehicle operator great latitude in maneuvering the vehicle. When maneuvering such a vehicle, it is desirable to be able to steer the vehicle with either the front or rear wheels. In these two modes of operation, one or the other set of wheels turns and the wheels at the opposite end of the utility vehicle remain fixed in a straight line orientation.
As noted in the afore mentioned '941 patent, it is also especially desirable to maneuver such a vehicle with both the front and rear wheels maneuvering in unison in a coordinated fashion. A so-called radius steering mode of operation causes the front and rear wheels to turn the same amount but in opposite directions to achieve a tight vehicle turning radius. In the so-called "crab" steering mode of operation, both front and rear wheels turn in unison in the same direction. This mode of maneuvering allows sideways movement of the vehicle. In combination the four modes of operation allow the operator to move the utility vehicle in an ingenious manner unavailable in prior art systems.
The apparatus for accomplishing the multi-mode steering of the '941 patent suggests utilization of tracking sensors to monitor the orientation of both the front and rear wheels and using a feedback control system to controllably regulate the orientation of the wheels during steering.